Photographic printing of digitized images is accomplished by modulating the intensity of a light beam as the beam moves relative to a photosensitive media. The most common motion pattern has the beam move rapidly along a single line as a single row of pixels is modulated or "clocked out" starting from a fixed margin location. The beam is then indexed to the next line of the page and returned to the margin before writing the next row of pixels.
A high degree of precision is required in identifying the margin location and pixel boundaries since misalignment of the pixels by as little as ten percent of a single pixel width degrades the perceptible quality of the printed image. Conventional printers have employed a two channel position encoding scheme to prevent misalignment. One channel of relatively high resolution is used to generate a pixel clock while a second channel generates one index pulse per revolution at a fixed position relative to signal line start. Synchronization of the pixel clock and the index to assure line to line uniformity is not an easy matter, and systems employ precise alignment of the two encoder channels and associated sensors to insure non-coincidence of pixel clock and line start signal transitions.
In view of the above, it is an object of the invention to provide a reliable low cost means for generating precisely aligned pixel clock and index signals for a digital scanning or printing system.